The present invention relates to a drum imagesetter and, more particularly, to a method for recording an image on a plate at the proper orientation.
A generally known type of imagesetter for recording a digital image onto a film or a printing plate is the so-called external drum imagesetter. Here, the film or the plate (both to be hereunder referred to as "plate") is attached to the cylindrical surface of the drum, then the drum rotates at a high speed, while one or more beams of light are modulated imagewise and focused onto the surface of the plate. According to one prevalent mode of operation, the assembly that projects the light beams onto the plate is made to move slowly and continuously parallel to the drum's axis. As a result, the focused light spot of each beam traces a helical path around the drum's surface. If the plate is mounted so that one of its edges is parallel to the drum's axis, the recorded trace of each beam is therefore inclined at some angle .alpha. from a direction normal to the edge, as shown in FIG. 1.
This inclination of the traced lines across the plate causes a corresponding distortion of the recorded image, namely a shear--or skew distortion. The effect is particularly strong, and thus objectionable, when the number of beams is relatively large, because then the pitch, which is the distance between consecutive traces of any one beam, and thus--the distance traveled by the beam axially during one drum rotation, is relatively large. The pitch is equal to the nominal distance between adjacent lines, multiplied by the number of beams.
It is noted that in common practice, the nominal distance between adjacent recorded lines (which is the inverse of the so-called resolution of the recording) may vary from one job to another. Consequently also the pitch of the helical trace varies and, accordingly, the angle .alpha. varies.
One known method, generally known in the art, to eliminate the resultant image distortion is to introduce a delay of the beam modulation signal, that regularly decreases by a slight amount from one rotation to the next. The effect of this varying delay is to cause the edge of the traced image, formed from the beginnings of all recorded lines, to be inclined at some angle .beta. to the aforementioned edge of the plate, as shown in FIG. 2. By choosing the proper magnitude of decrement of the delay, the inclination angle .beta. of the image edge can be made to be equal to the inclination angle .alpha. of the traced lines. The image is then formed on a truly rectilinear grid and thus is free from shear distortion.
A remaining problem is that the resultant image is rotated by an angle .alpha. with respect to the edge of the plate. Since, during printing, the plate is usually mounted on the print cylinder with its leading edge perpendicular to the paper's edge, the printed image likewise appears rotated by .alpha. with respect to the paper's edge--which, in most cases, is objectionable.
One solution to this problem is to mount the plate on the cylinder rotated by an angle -.alpha. with respect to its normal orientation, thus bringing the image back to be square with the paper. Such a solution would conceivably entail a specially adapted plate registration system for thus mounting the plate or modification of existing registration systems. This, however, may be difficult in practice--for the following reasons:
(a) A suitable mechanism would be complicated, since the value of .alpha. may be variable. PA0 (b) Installing or modifying a registration system in all affected presses may be expensive. PA0 (c) The orientation of the registration mechanism will have to be adjusted between different plates--e.g. for different values of .alpha. or for normally exposed (non-rotated image) plates which is time consuming and is prone to errors. PA0 (d) on some presses, a skewed plate may not be desirable. PA0 (a) punching at least one notch across the reference edge of the plate, PA0 (b) providing two stop pins, fixedly attached to the cylindrical surface, and PA0 (c) positioning the plate on the cylindrical surface so that one of the stop pins engages the notch. PA0 (a) punching at least two holes in the plate, each of the holes being symmetrically shaped, so that a line joining the centers of the holes is generally not parallel to the reference edge of the plate, PA0 (b) providing two registration pins, fixedly attached to the cylindrical surface, and PA0 (c) positioning the plate on the cylindrical surface so that the registration pins engage corresponding ones of the holes. PA0 (a) providing two curved surfaces, protruding normally from the cylindrical surface, such that essentially no line tangent to both of the curved surfaces is parallel to the geometric axis of the cylindrical surface, and PA0 (b) positioning the plate on the cylindrical surface so that the reference edge of the plate touches both of the curved surfaces, each at a respective contact point. PA0 (a) providing two registration pins, protruding from the cylindrical surface and matching the registration holes, such that a line joining the centers of their bases is generally not parallel to the geometric axis of the cylindrical surface, and PA0 (b) positioning the plate on the cylindrical surface so that the pins engage the holes. PA0 (a) punching a set of print registration holes in the plate, the centers of the holes being joined by a straight line and the line being rotated an angle .alpha. from its normal orientation with respect to the reference edge of the plate, and PA0 (b) providing means for aligning the reference edge of the plate with a line on the cylindrical surface that is essentially parallel to the geometric axis of the cylindrical surface.
There may also be objection to this solution on the grounds that
It is noted that plate registration systems on most presses usually employ a pin registration method. According to this method, a printing plate has a number of holes punched along its leading edge and the printing cylinder has corresponding pins rigidly attached thereto. When the plate is mounted on the cylinder, the pins engage the punched holes and thus the plate is positively oriented with respect to the cylinder.
Another solution to the problem is to mount the plate on the exposure drum of the imagesetter rotated by an angle .alpha. with respect to its normal orientation. The image will then be recorded at the correct orientation with respect to a reference edge (usually--the leading edge) of the plate. The main difficulty of such a solution is with mounting the plate at exactly the correct angle. Generally, plates are positioned on the drum, using registration pins attached thereto and engaging corresponding holes on the plate (which holes are subsequently used for registration in the press, as explained hereabove), or having the leading edge of the plate butt against a guide bar, or a set of pins, attached to the drum. Such positioning methods do not easily allow orienting the plate, during exposure, at the desired angle .alpha.--particularly if a is variable, as it often is. On the other hand, manually positioning the plate on the drum at the desired orientation may be a tedious and lengthy task, which is, moreover, prone to errors, and is therefore impractical.
There is thus a widely recognized need for, and it would be highly advantageous to have, a practical system for recording on a plate, in an external-drum imagesetter, a rectilinearly formed image so that, when printed, it will be squarely oriented with respect to the edge of the paper.